The present invention relates to controlled gas conditioning for a reaction gas of a fuel cell and a method for controlling gas conditioning for a fuel cell for the operation of the fuel cell.
Fuel cells are viewed as the energy source of the future, in particular also for mobile application in vehicles of any type. For this purpose, the proton exchange membrane fuel cell (PEMFC) has emerged as one of the most promising technologies, because it can be operated at low temperatures, offers high response times, and has a high power density and can be operated emission-free (reaction partners only hydrogen and oxygen). In addition, however, there is also a number of other fuel cell technologies, for example, an alkaline fuel cell (AFC), a direct methanol fuel cell (DMFC), a direct ethanol fuel cell (DEFC), a molten carbonate fuel cell (MCFC), a solid oxide fuel cell (SOFC), etc. A fuel cell uses one reaction gas in each case for the anode and for the cathode, for example, oxygen O2 (or air) and hydrogen H2, which react electrochemically to generate electrical current. The designs and the functions of the various fuel cells are sufficiently known, because of which they will not be discussed in greater detail here. A conditioning of the reaction gases is not absolutely necessary for operation of a fuel cell. However, the durability and performance and also power density, which are necessary for cost-effective and efficient use of fuel cells, for example, in a vehicle, can only be achieved with correct gas conditioning. It can be necessary in dependence on the type of the fuel cell to condition only one of the reaction gases or both reaction gases. The correct process control of a fuel cell, which includes in particular the gas conditioning, is decisive for the performance, durability, and the safe operation of the fuel cell, in particular also in the event of external and internal disturbances. In general, a faulty procedural process control results in a reversible or irreversible power loss (degradation) in fuel cells. An indicator for the present performance of a fuel cell is represented by the state of health (SoH). If a fuel cell reaches a defined value of the SoH (typically 80% of the continuous output in the new state in an automobile), this is referred to as end of service, which is undesirable and is to be avoided, of course.
For gas conditioning, the state variables pressure, temperature, and relative humidity (p, T, rH), and also the mass flow of the reaction gas are decisive.
For example, an excessively low mass flow results in a reactant deficiency, which immediately negatively influences the output and causes irreversible damage to the fuel cell depending on the duration and intensity. A further important influencing variable is the pressure of the reaction gas. A certain pressure gradient between anode and cathode does have a positive influence on the operation, but the membrane and thus the fuel cell are damaged in the event of an excessively large differential pressure. The relative humidity of the reaction gas represents a further example. In a proton exchange membrane fuel cell it is decisive, for example, to protect the membrane from drying out, since only a hydrated membrane conducts hydrogen cations and is thus efficient. However, a blockage of the gas channels and the diffusion paper by an excessive amount of liquid water, which results in reactant undersupply, also has to be avoided. In addition, a cyclic humidification and dehumidification of the membrane results in mechanical stress thereof and thus in turn in cracks and flaws (pinholes) in the membrane, which promote direct passage of hydrogen and oxygen. Both effects therefore again have a negative influence on the performance and state of health of a fuel cell. Not least, the temperature also plays a role. In addition to the accelerated chemical decomposition of the membrane at higher temperatures, the relative humidity and the temperature are also physically coupled, whereby the latter can also induce the above-mentioned effects. The mentioned examples only represent an excerpt of the possible effects in the event of deficient gas conditioning and are to serve for better comprehension of the problems.
The significant problem for gas conditioning is that the four mentioned influencing variables are dependent on one another because of physical (for example, thermodynamic) relationships and in addition have nonlinear behavior. This problem is often bypassed in that the components of the gas conditioning and the controlling concept for the gas conditioning are adapted to one another. Quite simple control based on characteristic maps, characteristic values, characteristic points, etc., together with simple controllers (for example, PID controllers) is thus largely sufficient. It is also possible that the parameters of the control (characteristic maps, characteristic values, characteristic points) are provided with correction factors in dependence on the SoH.
If one wishes to fully exhaust the possibilities of a fuel cell, such a simple control of the gas conditioning is often not sufficient. In particular, (highly) dynamic operation of the fuel cell (on the test stand or in the real application) generally cannot be implemented thereby. (Highly) dynamic operation is understood in this case in particular as a rapid response behavior of the control, i.e., the control is capable of following even rapid changes in the setpoint variables of the control with the least possible control deviation. Above all in the case of the development of a fuel cell on a test stand, where one generally wishes to subject the fuel cell to dynamic test runs (in the sense of the rate of change of the influencing variables, but also of the load of the fuel cell), in order to check or improve the behavior of the fuel cell, this is a problem.
Therefore, a controller which is capable of setting the controlled variables rapidly and accurately and above all also transiently is required for a dynamic control of the gas conditioning.
Various approaches for the control of the gas conditioning of a fuel cell are found for this purpose in the literature. Many of these approaches are based on a simplification to different extents of the thermodynamic relationships. Usually, only two of the mentioned influencing variables are controlled and assumptions are made for the other influencing variables. A suitable controller is then designed for this purpose. In most cases, the pressure or the humidity is controlled in this case. One example of this is Damour C. et al. “A novel non-linear model-based control strategy to improve PEMFC water management—The flatness-based approach,” Int. Journal of Hydrogen Energy 40 (2015), p. 2371-2376. A controller for the relative humidity using the known theory of differential flatness on the basis of a model of the membrane moisture is formulated therein. The flatness-based controller displays outstanding behavior of the setpoint value tracking, a high level of interference suppression, and a high stability. Nonetheless, not all influencing variables can thus be controlled, which makes this controller unsuitable for the intended control of the gas conditioning.
It is therefore an object of the present invention to specify controlled gas conditioning for a reaction gas of a fuel cell, and a corresponding control method for this purpose, which enable accurate and rapid control of influencing variables of the operation of the fuel cell.